1. Field of the Invention
The invention relates to a ventilator housing having a flow passage with an intake opening at one side and an outlet opening at the other side.
2. Description of the Related Art
Ventilators are often used for applications where a very uniform airflow, in particular, a laminar airflow, is to be provided across the entire flow cross-section. For example, in printing machines axial ventilators are arranged adjacent to one another, in particular, in a row arrangement, in order to supply large paper webs from above with an airflow after the printing process so that these paper webs can be placed or deposited more quickly onto a stack of paper webs. In this connection, a fluid-mechanical surface pressure is to be provided which is as high as possible and acts onto the web to enhance the force of gravity when placing the web onto the stack and thus push out the air at the underside of the web, present between the web to be deposited and the last-deposited web and impeding the placement of the web onto the stack. The deposition speed and deposition quality have a direct effect on the economic efficiency of the printing machine because they affect the speed of passage through the printing machine significantly. Because of this, an improvement of deposition and an increase of the deposition speed would cause directly an increase of the economic efficiency of the machine.
However, it was found especially for such ventilator applications that often the desired effect could not be achieved or achieved only to an unsatisfactory degree.
It is an object of the present invention to improve a ventilator housing of the afore described kind with respect to flow properties of the conveyed airflow, in particular, such that a ventilator provided with a ventilator housing according to the invention is suitable particularly for the above described use in printing machines.
In accordance with the present invention, this is achieved in that outflow contouring means are arranged in the area of the outlet opening so as to surround the flow cross-section of the outlet opening and are configured such that a vortex formation (large scale, low frequency) is reduced at least to some degree in a shearing layer, which surrounds the actual airflow resulting from ventilator operation and located between the airflow and the surrounding, usually stationary, air.
The invention is based on the recognition that behind each ventilator, naturally, a free jet is formed which mixes only at a certain distance with the surrounding air and/or with the free jet of neighboring ventilators. These free jets have the property that in their center a relatively low degree of turbulence is present. Toward the sides, the moving air which has been blown out mixes with the attracted stationary air within a shearing layer (entrainment effect). In this shearing layer, there exist almost exclusively large scale, low frequency vortex shapes. Because of their high energy contents, these vortex shapes have great longevity. In regard to the above described preferred applications, this has the result that the shedding of the vortices causes the printed web to be excited to perform vibrations at its resonant frequency. In certain situations, the resonant frequency of the paper webs correlates with the vortex shedding frequency. This mechanism prevents an increase of the deposition speed and can furthermore result in temporary disruptions of the deposition process.
By means of the outflow contouring means according to the invention, which are preferably serrated and arranged in a crown-shape about the outlet opening of the ventilator, the large-scale vortices in the shearing layer are broken up or their formation is prevented from the beginning. The excitation energy of the airflow onto the paper web in the preferred application of printing machines is thus drastically reduced and a corrugation of the paper web is prevented or at least reduced. The important advantages are thus as follows:
stabilization of the free air jet,
reduction of the large scale vortices in the edge area of the free air jet,
reduction of low-frequency excitation components,
noise advantages.
Further advantageous features of the invention will be described in the following.